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I want to buy bulk activated carbon for use in a DIY air-filtration set-up, like this, to improve air quality in my bedroom.

From what I understand, the relatively larger sizes of carbon media on the market are optimal for air filtration to maximize the amount of time the air is in contact with the carbon.

My question is, once my activated large-granule carbon is used up, say after a few months in my air filter, am I throwing away any untapped potential? Specifically, is there value in further crushing/grinding the carbon and then putting it into another application optimized for the new, more fine-grained size-- perhaps a water filter, or an air filter where the carbon is in a long tube, and air is moving slowly through it.

The same FAQ suggests it is impractical and messy to grind up the AC, but it seems to me that mess can be contained with a bit of planning. I'm also happy to be pointed to general resources that are helpful, especially if I'm barking up the wrong intuition trees, here. Thanks!

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Activated carbon is called "activated" when its grains contain a large amount of large cavities. These cavities are big enough to hold large molecules of undesired substances (oils, smokes) which are present in the air. Only small molecules like oxygen and nitrogen from the air can go through without being retained. So the activated carbon removes pollution from the air. After a long time, there are more and more cavities occupied by these unwanted substances. As a consequence, the effectiveness of the activated carbon slowly decreases. It has to be replaced. But it is no use grinding it. If one wants to reuse this carbon again, one should heat it up to such a high temperature that the substances fixed in the cavities are vaporized. Unfortunately some substances like soot will never get vaporized. So the effectivity of the activated carbon will never be entirely restored

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  • $\begingroup$ Thanks! Just to clarify, all potential cavities/sites to hold large molecules are already accessible, even at a coarse grain? No new, perhaps inner cavities are exposed by breaking down the carbon some? If so, is it because the process of activation only engages the initial surface area? $\endgroup$
    – Mark K
    Apr 30 '21 at 20:36
  • $\begingroup$ @MarkK Sorry I do not fully understand what you say. when writing "the process of activation only engages the initial surface area". $\endgroup$
    – Maurice
    May 2 '21 at 14:05
  • $\begingroup$ Sorry, yes. I'm trying to understand why grinding carbon into smaller pieces is ineffective. to me, after the activated carbon has all it's cavities filled, grinding is a way to expose new cavities of the activated carbon, that are now new binding sites. If grinding is ineffective, then am I to understand that: all initially available cavities on the activated carbon, in the size of granule it is, are the only cavities available, even if new surfaces are exposed by breaking pieces into smaller ones? It makes sense to me if the activation process works only on initially exposed surface of C. $\endgroup$
    – Mark K
    May 2 '21 at 17:35
  • $\begingroup$ Your answer implies what I'm looking for, can I ask for clarification? The cavities available for capturing unwanted particles, are they only accessible on the exterior surface, or is the carbon structure open / exposed enough that cavities throughout it can also capture particles? If cutting does nothing, then I assume carbon is like a sponge with water analogous to the unwanted particles. In my question I am wondering if the particles behave like a viscous grease that can only be captured by the surface of the sponge. <- happy to update question with this if it is helpful. $\endgroup$
    – Mark K
    Dec 8 '21 at 13:35
  • $\begingroup$ The cavities were initially cells. Wood is a living system, made of cells containing a nucleus, cytoplasm, etc. surrounded by a membrane. When overheating wood, water will evaporate, and the membranes are destroyed. Later on, all organic stuffs aretransformed into smoke, tar and other volatil substances. The membrane itself is transformed into charcoal or impure carbon. So the obtained "activated carbon" is made of broken spheres, linked up with tiny channels. The adsorption of pollution is not an effect of surface. It occurs in the bulk of the activated carbon . $\endgroup$
    – Maurice
    Dec 8 '21 at 19:31

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